Arminius (Irmin or Hermann) of the Germanic Cherusci tribe. An alliance of Germanic tribes under his command defeated the imperial Roman army at the battle of Teutoberg Forest in 9 C.E.
The last battle of the Roman Empire was in 451AD between an alliance of Rome (which was failing) and the Visigoths in France (Gaul) against Attila the Hun and the Huns. The Romans won a strategic and decisive victory, and later the Huns were wiped out by the Teuton tribes after losing this battle. Although it was a major victory it did not stop Rome from collapsing and finally being sacked in 476. Rome staggered on for some time afterwards but it was never the same or powerful again.